The invention relates to the detection of a tone signal with predetermined characteristics in the presence of a high degree of noise.
Methods for the extraction of weak signals in the presence of noise have been developed in the space age, particularly for detecting weak signals transmitted to earth from space probes situated at planetary distances from the earth. These methods have made extensive use of pseudo-random sequences; the noise dealt with has, however, almost exclusively been "white noise"; "DATA TRANSMISSION" by William R. Bennet, and James R. Davey (McGraw-Hill Book Co., 1965), provides a good analysis of such transmission methods.
Treatment of digital space communications, plus coverage of underlying theoretical concepts, including material on pseudo-random codes can be obtained from "Digital Communications with Space Applications", Solomon W. Golomb et al. (Prentice-Hall series, 1964).
In addition to proposals for methods and apparatus for extracting a weak signal in the presence of noise, various proposals have been made for detecting the presence of a speech signal in a voice channel. For example, U.S. Pat. No. 4,057,690, dated Nov. 8, 1977, provides an active channel indication when the presence of a speech signal is detected in the voice channel; an idle channel indication is obtained when the absence of the speech signal is detected in the same voice channel.
German Offenlegungsschrift No. 26 23 025, published Jan. 27, 1977, deals with a method and an apparatus for analysis of a speech signal, including multi-channel signals, in which an estimated value is compared to another predetermined signal value, the signal itself being compared to a second constant prearranged threshold value, these various values being further processed so that a logical signal is obtained which corresponds to the presence or absence of a voice signal in a speech channel.
There is also known a ripple control receiver for use in a ripple control system, wherein signals are superimposed on an electrical power distribution, for example, for control purposes. Cf. British Pat. No. 1,221,179, published Feb.3, 1971. In that British patent, there is provided a ripple control receiver, including an input circuit including a frequency filter, an amplifier and a receiving relay having operating means operable by direct current. The output of the filter is connected directly to the control input of the amplifier, which has a response threshold, and is blocked in the rest condition of the ripple control receiver. The operating means of the receiving relay are connected directly in the operating circuit of the amplifier, and the voltage transfer of the filter has substantially the same temperature coefficient as the response threshold of the amplifier.
There is also known a mechanism for automatic recognition and differentiation of complex information, particularly to determine the modulation-type of a high frequency signal. Cf. German Offenlegungsschrift No. 25 55 248, published June 16, 1977. This publication teaches a preliminary circuit in which the information is continuously subdivided into a plurality of basic criteria, a coding circuit which codes such criteria, and stores them in an intermediate memory, a control circuit which transfers the stored and coded criteria into a plurality of classification vectors, which in turn are stored at predetermined storage locations corresponding to respective classes of a second memory as a sample corresponding to the frequency of occurrence of classification vectors, which vectors are in turn fed to an M-ary shift register, in which the last classification vectors are stored and by means of which the "frequency of occurrence" histogram has been formed in the second memory. There is further provided, in that publication, a comparison circuit in which predetermined patterns are compared with the patterns stored in each memory, an output signal being obtained upon coincidence of reference values with those stored in the memory.
The publication, however, deals purely with the recognition of various predetermined waveforms and criteria, relating primarily to the frequency content of a received signal, without taking into account other important criteria of the signal. The publication, furthermore, in contrast to the instant invention, is not related to the extraction of a signal in the presence of noise, nor does it provide counting means for signaling the presence of an electrical signal, when a plurality of predetermined estimates in one of at least two classes exceeds a predetermined number.
There are also known detectors to determine the presence of a tone frequency signal, so-called tone detectors, which mostly use a band pass filter tuned to the expected tone frequency.
An evaluation circuit coupled to the band pass filter determines whether an envelope of the tone frequency signal at the output of the band pass filter is below or above a predetermined threshold, and correspondingly an output is obtained, which is either a "YES" or a "NO" signal, signifying the presence or absence of the tone signal.
Such tone detectors are frequently used in remote-controlled receivers of diverse types, which receivers are connected to a transmitter, either by a dedicated line or by radio. Such detectors operate satisfactorily, as long as any corrupting signals at their inputs consist of white or thermal noise only. A reliable YES/NO signal is not, however, insured, if corrupting impulse signals or interference signals of a small bandwidth are superimposed onto the tone frequency signal.
There are also known detectors for determining the presence of a direct current or direct voltage, which consist of an integrator and a threshold circuit. Such detectors serve for the reception of alarm signals in alarm circuits, where the alarm signals are transmitted by direct current. Any erroneous signal provided by such a detector due to interfering signals may have serious consequences.